Quadrature clocks are widely used in many applications such as a phase interpolator and a half-rate clock data recovery (CDR) circuit. The generated quadrature clocks are required to be in symmetrical or equally divided phases. In other words, each clock period needs to have equal high and low periods, or an approximately 50% duty cycle. In a rectangular waveform, a 50% duty cycle can be described as the percentage of the full cycle that the signal remains at logic 1. For instance, if the signal spends half the clock period at logic 1 and the other half at logic 0, then the waveform generated would be a waveform with a 50% duty cycle.
Usually, the 50% duty cycle is achieved by doubling the clock rate or increasing the frequency of the clock rate before dividing the clock rate by 2. However, in high speed designs where the output clocks are already running at a high frequency, e.g., 5 GHz and above, such a method is not feasible because it is very difficult, or even impossible, to double an already high frequency clock. Furthermore, by dividing the clock rate, this method would slow down the system clock considerably. Another method is to couple a pair of positive feedback inverters to a pair of clocks running at opposite phases. However, this method is inadequate because it has a limited correction range and the inverters used invariably increase the power consumption of the circuit.
The duty cycle of each clock also depends on the process, temperature and supply voltage. Therefore, clock duty cycle correction circuits or methods need to perform consistently across a wide range of processes, voltages and temperatures, i.e., duty cycle correction circuits that correct output clocks to 50%, or close to 50%, duty cycle should be reliable across all PVT (Process, Supply Voltage, Temperature) corners.
Therefore, it is desirable to have a consistent duty cycle correction or clock tuning circuit that that does not necessarily speed up or slow down the clock rate and can correct severe duty cycle offsets without a significant increase in power consumption. It is within this context that the invention arises.